Coherently combined laser arrays having large numbers of laser channels are of interest since they can be used to obtain high power laser beams. There are two general approaches used to obtain such a combination of coherent laser beams. The first approach is tiled beam combining, and the second is filled aperture beam combining. In the tiled beam approach, lasers to be combined are laid side by side and pointed in the same direction such that they co-propagate as a single beam. Whereas, in the filled aperture beam approach, a beam splitting optical system is used in reverse to combine the beams so that the combined beams lie on top of one another and co-propagate as a single beam. Gaps between beams occur with the Tiled Beam approach as it is impossible to squeeze two adjacent beams so close together that no dead space occurs between them regardless of the tiling optics used. The dead space degrades the peak brightness that can be achieved after the beam has propagated into the far field. A filled aperture approach can, however, eliminate these tiling gaps thereby enhancing the peak brightness of the beam.
These arrays all require some means of actively controlling the phases of the individual lasers in order to synchronize their phases. Active laser phase control feedback methods generally fall into one of three categories: (1) Heterodyne as reported in J. Anderegg, S. Brosnan, E. Cheung, P. Epp, D. Hammons, H. Komine, M. Weber, and M. Wickham, “Coherently coupled high-power fiber arrays,” in Fiber Lasers III: Technology, Systems, and Applications, A. Brown, J. Nilsson, D. J. Harter, and A. Tunnermann, eds., Proc. SPIE 6102, 61020U-1 (2006), (2) Synchronous Multi-dither (LOCSET) as reported in U.S. Pat. No. 7,058,098, U.S. Pat. No. 7,187,492, and U.S. Pat. No. 7,223,433, and (3) Stochastic Parallel Gradient Descent (SPGD) which is usually referred to as Hill Climbing as reported in M. A. Vorontsov and V. P. Sivokon, “Stochastic parallel-gradient-descent technique for high-resolution wave-front phase-distortion correction,” J. Opt. Soc. Am. A 15, 2745-2758 (1998). These three approaches differ in the method by which they develop error signals for feedback. The Heterodyne approach first combines a frequency-shifted reference with the tiled output beam and then measures the phase of the resultant beat signals for each tile. In the Synchronous Multi-dither approach each beam is tagged with a small phase dither and a single detector senses the combined output beam. Radiofrequency (RF) mixers are then used to isolate each phase from the combined output beam. In the Hill Climbing approach, the phase of each beam is changed in a digital step, and the resultant combined beam power is then observed to determine whether the combined far field power increased or decreased. Results of these three approaches are fed back to control the phases of the individual lasers.
To phase-lock large numbers of laser channels, it is necessary to select a phase-locking approach that is scalable while maintaining low parts count and minimizing system complexity. All phase-locking methods have intrinsic limits to their channel count scalability, which ultimately limits the achievable closed-loop control bandwidth. For the purpose of scaling phase control to large channel counts with large closed-loop control bandwidth, it has been found advantageous to organize and control lasers in groups or sub-nests. The individual lasers in each group are phaselocked together using one of the methods described above, and subsequently the groups are locked together to lock the overall array. This nesting approach can be extended to multiple levels of nesting. The advantage of nesting is that it provides a means to increase channel count scalability in the laser array by limiting the number of channels in any one group to preserve the closed-loop control bandwidth of the underlying phase-locking control method.